This invention relates to stepped voltage electric power supplies including battery cells as the prime source of electric power and especially to such power supplies adaptable for use in powering motor vehicles. It is well known in an electric powered motor vehicle to vary the speed of the vehicle by controlling the voltage of the power supply applied to the vehicle's motor; and a particularly simple way of accomplishing this with a multi-celled power supply is to vary the number of cells connected in series and thereby vary the output voltage in step-wise fashion. Unfortunately, the direct application of a straight-forward switch in this application leads to the premature discharge of those battery cells switched in first at low speed compared with those battery cells which are not switched in until high speeds, since the former are used more than the latter. Realization of this fact has led to the suggestion of a number of rather complex and awkward switching schemes to vary the cells used in such switching apparatus and thereby equalize the discharge of the cells.